Has_and_belongs_to_many for the same model

Rails
1

Rails 3.1.3

I am wondering if has_and_belongs_to_many association can be used for a
single model?

More specifically, say I have a model called, Person, another model
called Meeting.

Person 1 and Person 2 share some kind of association through Meeting so
that

person1 = Person.find(…)

then,

person1.meeting returns person2.

If my understanding is correct, has_and_belongs_to_many association is
used for one model with another.

Somehow I want to find a model object from the same object.
Do you think we can do that?

soichi

2

On Wed, Nov 28, 2012 at 9:33 PM, Soichi I. [email protected]
wrote:

Somehow I want to find a model object from the same object.
Do you think we can do that?

https://www.google.com/search?q=rails+self+referential

–
Greg D.

3

On Wed, Nov 28, 2012 at 9:33 PM, Soichi I. [email protected]
wrote:

I am wondering if has_and_belongs_to_many association can be used for a
single model?

Circular associations are possible, I do it all the time for comment
systems.

4

rails self referential - Google Search

Thanks. This is what I wanted.

But in my case, two model objects (in the RailsCasts, User) are equal.
So, I tried

in person.rb

has_many :give_meetinges, :class_name => "Meeting", :foreign_key =>

“give_id”
has_many :gives, :class_name => “Person”, :through =>
:give_meetinges, :source => :person
has_many :take_meetinges, :class_name => “Meeting”, :foreign_key =>
“take_id”
has_many :takes, :class_name => “Person”, :through =>
:take_meetinges, :source => :person

in meeting.rb

belongs_to :give_person, :class_name => "Person"
belongs_to :take_person, :class_name => "Person"

in the view, (‘take’ is person object)

    <%= take.give_meetings.give_person %>

raises an error,

undefined method `give_plan’ for []:ActiveRecord::Relation

Since it’s ‘Relation’,

    <%= take.give_meetings.first.give_person %>

also raises an error

undefined method `give_plan’ for nil:NilClass

Can see problems in my code?

soichi

5

i don’t see any give_plan on your code.

Sorry, I pasted a wrong portion of code error.

it’s

undefined method give_person' for []:ActiveRecord::Relation undefined methodgive_person’ for nil:NilClass

6

On Thu, Nov 29, 2012 at 4:29 PM, Soichi I. [email protected]
wrote:

“give_id”
has_many :gives, :class_name => “Person”, :through =>
:give_meetinges, :source => :person
has_many :take_meetinges, :class_name => “Meeting”, :foreign_key =>
“take_id”
has_many :takes, :class_name => “Person”, :through =>
:take_meetinges, :source => :person

I’m not sure if you need to specify the class_name for a has_many
through
association

in meeting.rb

belongs_to :give_person, :class_name => "Person"
belongs_to :take_person, :class_name => "Person"

this means that the foreign key is set to give_person_id and
take_person_id
but in your person model, the foreign key is give_id and take_id

in the view, (‘take’ is person object)

    <%= take.give_meetings.give_person %>

raises an error,

undefined method `give_plan’ for []:ActiveRecord::Relation

Since it’s ‘Relation’,

    <%= take.give_meetings.first.give_person %>

also raises an error

undefined method `give_plan’ for nil:NilClass

this is an expected error if there are no give_meetings for take. if
there
is at least on give_meetings for take, this will not raise an error
(assuming
that your associations and foreign keys are setup correctly)

i don’t see any give_plan on your code. paste the code that stacktrace
points to the source of this error.

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–

7

in meeting.rb

belongs_to :give_person, :class_name => "Person"
belongs_to :take_person, :class_name => "Person"

this means that the foreign key is set to give_person_id and
take_person_id
but in your person model, the foreign key is give_id and take_id

This was the problem.
I changed them to give_person_id and take_person_id and now it works!

Thanks for your help!

soichi